Field of the Invention
In the fabrication of microchips, semiconductor substrates are patterned using lithographic processes. For this purpose, a film of light-sensitive resist is applied to the substrate and the film is then exposed in sections. The exposure in sections can be effected by direct writing with an electron beam or as a result of a mask which includes the necessary information concerning the pattern to be formed being disposed in the beam path between the light source and the photoresist. Exposure using a photomask is generally carried out using short-wave, mono-chromatic light which is generated, for example, using a laser. Currently, industrial fabrication of microchips uses light with a wavelength down to as little as 193 nm. There is currently ongoing research aimed at adapting the lithographic processes to use even shorter wavelengths, in particular 157 nm. The exposure results in a chemical modification of the photoresist, with the result that selectively only the exposed or unexposed sections of the resist film become soluble in a developer. Then, in a developing step, the soluble sections of the resist film are removed and the substrate disposed beneath the resist film is uncovered. The insoluble sections of the resist form a resist pattern which protects the substrate disposed beneath the resist film from attack, for example from etching media. A distinction is drawn between positive photoresists, in which the exposed sections of the resist film are converted into a soluble form, and negative photoresists, in which the exposed regions are converted into an insoluble form, for example by cross-linking of the polymers contained in the resist and remain on the substrate as a patterned resist after the developing step. To prevent exposure errors resulting from reflection, an anti-reflection film that absorbs the incident radiation, for example by interference, is generally disposed beneath the photoresist film. The antireflection films are generally applied to the substrate by sputtering, requiring accurate control of the layer thickness of the antireflection film that is to be produced. After the resist film has been patterned, the anti-reflection film or the substrate in the trenches of the resist pattern is uncovered and can be removed using an etching medium. The etching operation therefore transfers the pattern that has been predetermined by the patterned resist to the substrate. By way of example, wet-chemical processes can be used for this purpose. Particularly in the case of patterns with very small dimensions or patterns that are etched very deeply into the substrate, such as for example trenches for deep trench capacitors, however, plasma is generally used for the etching. The plasma may have a preferential direction, with the result that the substrate can be etched anisotropically. This also allows the pattern which is predetermined by the patterned resist to be transferred to the substrate to scale, making it possible to produce trenches with a depth of up to 50 μm. Relatively long etching times are required to produce patterns of this type. Therefore, the patterned resist must have a sufficient etching resistance to ensure that it still has a sufficient layer thickness for the pattern to be transferred to the substrate without defects even toward the end of the etching operation. However, this presents considerable difficulties. To allow accurate reproduction of the pattern to be produced in the photoresist film, the latter is configured to be very thin, in order to ensure sufficient depth of focus. Therefore, the pattern produced from the resist film likewise has a low layer thickness. Therefore, the pattern produced from the photoresist is generally not sufficiently able to withstand etching plasma. Furthermore, the photoresists generally contain organic polymers as their main component. In an oxygen plasma, the polymers can easily be oxidized to form carbon dioxide and water. Therefore, the mask would be worn away very quickly, and consequently the pattern cannot successfully be transferred to the substrate. To overcome these difficulties, the mask produced from the photoresist is used to produce a mask made from a material which has a high stability with respect to the plasma used to etch the substrate. For this purpose, a layer of a hard-mask material is applied to the substrate that is to be etched. The thickness of the layer is selected to be relatively great in order to ensure sufficient stability of the hard-mask material with respect to the etching plasma while the substrate is being etched. An antireflection layer is applied to the hard-mask layer, for example by sputtering. Finally, a layer of a photoresist is applied to the antireflection layer. Then, as described above, the photoresist film is exposed and developed, so that a patterned resist is obtained. Once again, the antireflection layer is uncovered in the trenches of the patterned resist. Then, the antireflection layer is removed in the trenches using plasma, and subsequently the pattern is transferred to the hard mask layer using suitable etching plasma. Finally, the patterned resist is removed, for example by being dissolved in a solvent. A pattern containing a hard mask material which, compared to the etching resistance of the photoresist film, has a considerably higher stability with respect to the plasma used to etch the substrate, is then present on the substrate. Next, the substrate material that is uncovered in the trenches is removed using suitable etching plasma, and in this way the pattern that has been predetermined by the hard mask is transferred to the substrate. Finally, the hard mask is removed, for example using suitable plasma or by being dissolved using a suitable solvent.
The plasma steps required in order to deposit and pattern the layer of the hard mask material makes the use of a hard mask of this type time-consuming and expensive. However, the fabrication of microchips is subject to high pressure on costs, since a considerable price drop starts relatively quickly after a new micro-chip has been introduced to the market. Therefore, there is a constant demand for further development of the fabrication processes in order to simplify the fabrication of microchips and therefore allow this process to be carried out at lower cost.